During CVD processing, deposition gases are released inside a processing chamber to form a thin film layer on the surface of a substrate being processed. Unwanted deposition on areas such as the walls of the processing chamber also occurs during such CVD processes. Because the residence time in the chamber of individual molecules in these deposition gases is relatively short, however, only a small portion of the molecules released into the chamber are consumed in the deposition process and deposited on either the wafer or chamber walls.
During semiconductor manufacturing processes in which CVD is utilized to form layers on wafers, it would be ideal if the injected process gas would deposit only on the wafer substrate surface, however, in reality, some gas molecules miss the substrate surface and deposit on the process chamber surfaces. Some of the unconsumed gas molecules are pumped out of the chamber along with partially reacted compounds and reaction byproducts through an exhaust line under vacuum. Many of the compounds in this exhausted gas are still in highly reactive states and/or contain residues or particulate matter that can form unwanted deposits in the exhaust line. In a processing chamber such as an Epi Centura® chamber manufactured by Applied Materials, the temperature of process gases falls dramatically upon exit from the processing chamber as the process gases enter the exhaust line, resulting in coating of the exhaust inserts, exhaust cap, and at least the first four feet of the exhaust line. In addition to the materials described above, the coating has been observed to be generally a translucent viscous liquid, with a honey-like consistency. The condensed exhaust byproduct can also appear opaque white to opaque yellow to opaque reddish brown, depending upon the process conditions and location in the exhaust line. When the condensed exhaust byproduct is opaque, it appears to be in a solid phase. It is believed that the translucent liquid reacts immediately on exposure to the ambient to form an opaque white material.
Thus the buildup of liquid and solid material in the exhaust lines poses several problems. First, the build-up poses a safety threat in that the matter is often a pyrophoric substance that may ignite when the vacuum seal is broken and the exhaust line is exposed to ambient conditions during standard, periodic cleaning operations. Second, if enough of the deposition material builds-up in the exhaust line, the exhaust line and/or its associated vacuum pump may clog if it is not appropriately cleaned. Even when periodically cleaned, matter build-up interferes with normal operation of the vacuum pump and can drastically shorten the useful life of the pump. Also, the solid matter may backwash from the exhaust line into the processing chamber and contaminate the processing chamber. If the translucent liquid is rapidly exposed to air, an explosive reaction can occur.
To avoid these problems, the inside surface of the exhaust line is regularly cleaned to remove the deposited material. This procedure is performed during a standard chamber clean operation that is employed to remove unwanted deposition material from the chamber walls and similar areas of the processing chamber. Common chamber cleaning techniques include the use of an etching gas, such as fluorine, to remove the deposited material from the chamber walls and other areas. The etching gas is introduced into the chamber and a plasma is formed so that the etching gas reacts with and removes the deposited material from the chamber walls. Such cleaning procedures are commonly performed between deposition steps for every wafer or a number of wafers.
Removal of deposition material from chamber walls is relatively straight forward in that the plasma is created within the chamber in an area proximate to the deposited material. Removal of deposition material from the exhaust line is more difficult because the exhaust line is downstream from the processing chamber. In a fixed time period, most points within the processing chamber come in contact with more of the etchant fluorine atoms than do points within the exhaust line. Thus, in a fixed time period, the chamber may be adequately cleaned by the clean process while residue and similar deposits remain in the exhaust line.
To attempt to adequately clean the exhaust line, the duration of the clean operation must be increased. Increasing the length of the clean operation, however, is undesirable because it results in equipment downtime, which adversely affects wafer throughput. Also, such residue build-up can be cleaned only to the extent that reactants from the cleaning process are exhausted into the exhaust line in a state that they may react with the residue in the exhaust line. In some systems and applications, the residence time of the exhausted reactants is not sufficient to reach the end or even middle portions of the exhaust line. In these systems and applications, residue build-up is even more of a concern.
Several different devices have been designed to facilitate the cleaning of such exhaust lines. One approach that has been employed to clean the exhaust line is to trap the particulate matter present in the exhaust stream before it reaches the vacuum pump by diverting gas flow into a collection chamber from which particulate matter cannot easily escape. Devices that rely on this technique provide a removable door or similar access to the collection chamber so that once a sufficient amount of material has built up within the chamber it can be easily removed. Typically, the substrate deposition system is temporarily shut off during the period in which the collection chamber is cleaned, thereby limiting or reducing wafer throughput of the system.
One approach that has been employed to clean the exhaust line relies on a scrubbing system that uses plasma enhanced CVD techniques to extract reactive components in the exhaust gas as film deposits on electrode surfaces. The scrubbing system is designed to maximize the removal of reactants as a solid film and uses large surface area spiral electrodes. The spiral electrodes are contained within a removable canister that is positioned near the end of the exhaust line between the blower pump and mechanical pump. After a sufficient amount of solid waste has built up on the electrodes, the canisters may be removed for disposal and replacement.
Problems exist in this prior art method in that the system relies on the large surface area of the electrodes to provide an area for deposited solid matter to collect. To accommodate the large surface area of the electrodes, the system is necessarily large and bulky. Furthermore, extra expenses are incurred in the operation of this prior art scrubber system since the removable canister is a disposable product that must be replaced and properly disposed. Also, the scrubbing system is located downstream from a beginning portion of the vacuum exhaust line and thus does not ensure removal of powdery material or particulate matter that builds-up in this portion of the line.
Another approach to cleaning the exhaust lines utilizes what is sometimes referred to as a point of use reactor. The point of use reactor uses a heater cartridge to react excess gas from the process chamber. The maximum temperature of the point of use reactor is about 500° C., and reaction byproducts remain in the exhaust line. The point of use reactor is not effective for reduced pressure deposition since the polysilicon formation causes significant particle formation.
Still another method and apparatus for cleaning the exhaust line involves trapping powder residue and other particulate matter in a collection chamber and removing the same with a plasma formed downstream of the reaction chamber. Constituents from the plasma react to form gaseous products that are readily pumped through and out the exhaust line. The conversion process relies on forming a plasma from an etchant gas in the area where the particles are trapped, and this type of apparatus is sometimes referred to as a Downstream Plasma Apparatus or “DPA” for short. Several examples of such an apparatus and method are described in commonly assigned U.S. Pat. No. 6,194,628, which is incorporated herein by reference in its entirety. One embodiment of the apparatus described in U.S. Pat. No. 6,194,628 includes a coil surrounding a gas passageway defined by a vessel chamber. The coil is connected to an RF power supply that is used to excite molecules from particulate matter and residue within the passageway into a plasma state. The RF power in a commercial version of such an apparatus utilizes high frequency RF power with a fluorine-containing gas such as nitrogen trifluoride to chemically etch the exhaust deposit. The upper limit of the frequency range of the power supply described in U.S. Pat. No. 6,194,628 is listed as 200 MHz, and the frequency used in experimental setup in U.S. Pat. No. 6,194,628 is 13.56 MHz. A potential problem with the use of a fluoride-containing gas is compatibility with materials in the reactor, disposal of hazardous waste generated by the cleaning process, and damage to the equipment if proper controls are not employed.
Accordingly, it would be desirable to provide methods and apparatus for efficiently and thoroughly cleaning the exhaust line in a semiconductor processing systems.